Orthodontic-induced Temporary Sensory Nerve Deficit of the Lower Lip

Case report

History

A medically fit 13-year-old male presented to their orthodontist with Angle's Class II division 1 occlusion on a skeletal 1 base with average vertical proportions, 6-mm overjet, moderate crowding of upper labial segment with buccally displaced maxillary canines and right posterior buccal crossbite (Figure 1).

Pre-treatment dental panoramic radiograph (DPR) revealed the presence of all permanent teeth with absence of pathology (Figure 2). Unerupted LR7 had mild mesio-angular impaction with curved mesial and straight distal roots superimposed over the mandibular canal.

Treatment

Orthodontic treatment was carried out in primary care on a non-extraction basis with upper and lower fixed pre-adjusted edgewise appliances with MBT (0.022” x 0.028” slot size) prescription.

18 months into treatment, closed exposure and attachment of a gold chain to the buccal surface of unerupted LR7, and extraction of lower right third molar was carried out under general anaesthetic. This was to facilitate disimpaction and uprighting of LR7.

Traction to LR7 gold chain was commenced by the referring orthodontist 3 weeks after-surgery using Z-spring from gold chain to lower 0.019” x 0.025” stainless steel archwire.

28 months into treatment, molar tube was positioned in an optimal position buccally on LR7 and engaged with 0.016” x 0.016” nickel titanium archwire 7 weeks later.

6 weeks later, the patient reported ‘numbness’ to the right side of their lower lip.

Palpation over the lower half of the face isolated numbness to the right side of the lower lip with no midline crossover. A DPR taken showed partially erupted LR7 with an improved and more distal angular position, although with apices of the mesial and distal roots still superimposed over the mandibular canal (Figure 3). The 0.016” x 0.016” nickel titanium archwire was disengaged, and the patient was referred back to the Liverpool dental hospital.

Investigation

The patient was reviewed in the oral surgery department 3 weeks later (Figure 4). The patient reported resolution of the ‘pins and needles-like numbness’ over the right side of the lower lip 2 weeks after onset. Pinprick test identified a normal response with no sign of sensory nerve deficit.¹⁰ A provisional diagnosis of paraesthesia was made, based on the patient's account of matching that of an ‘abnormal sensation, which can be spontaneous or evoked’ as per the International Association for the Study of Pain.⁹

CBCT revealed a mesio-angular and infra-occluded LR7 with a straight distal root and S-shaped mesial root (Figure 5a). The mandibular canal crossed the distal root in the apical third region lingually with narrowing and no intervening bone (Figure 5b). It then crossed the apical third region of the mesial root with approximately 1 mm of intervening bone (Figure 5c).

Outcome

LR7 was omitted from further treatment as advised by a consultant orthodontist in the Liverpool dental hospital. The patient's orthodontic treatment was completed uneventfully 3 months later with omission of the LR7. LR7 was unchanged at debond and the patient was discharged with upper and lower Hawley retainers (Figure 6).

Discussion

Sixteen previous cases of sensory nerve symptoms during orthodontic treatment have been reported (Table 1). These have involved patients aged between 12 and 57 years, with 10 females, five males and one unrevealed gender. All cases were associated with mandibular teeth; eight with second molars, five with second premolars, two with wisdom teeth, and one attributed to the lower right posterior teeth.

Most frequent onset of altered sensation followed an increase in archwire size and movement of roots in a direction towards the mandibular canal. Anatomical variations of the course of the mandibular canal, its neurovascular structures and surrounding mandible exist, and these may be a risk factor for sensory nerve symptoms.

Typically, the mandibular canal follows its course bucco-lingually from the mandibular foramen in contact, or close proximity, to the lingual cortical plate in the region of the mandibular molars before crossing buccally and anteriorly to exit the mental foramen in the region of the mandibular premolars.^11,12

Ozturk et al characterized variations in the emergence patterns of the mandibular canal in the bucco-lingual plane as sharp, soft-curved and straight exits. Similarly, variations in the course in the vertical plane have also been differentiated into three types: straight (12.2%); progressive descent (36.7%): and catenary-like (51.1%).¹²

It is more common for premolar roots to be in close proximity to the mandibular canal. The medial-lateral trabecular space between the buccal and lingual cortical plates reduces towards the mandibular premolars. As a result, the mandibular canal occupies more trabecular space, reducing the amount of intervening bone and increasing the proximity of the apices of the premolars to the canal.^11,12

Anatomical variation is also more frequent in the presence of impacted or ectopic teeth, long or large roots, and with decreased mandibular body height.¹³ In the premolar region, the mental foramen assumes a more superior position from the inferior border of the mandible with increased height of the body.¹²

A CBCT study looking at the relationship between the mandibular canal and mandibular teeth exhibited decreasing distance between the canal and apices from premolars to molars. This was more significant in younger subjects and females. Direct contact relationships were found in 1.6%, 3.3%, 3.3%, 16% and 32.2% of teeth running from mandibular first premolars to third molars, respectively.¹⁴ Interestingly, most cases in Table 1 involve females, younger patients and molars.

All cases in Table 1 had imaging that identified a close relationship of the offending tooth apices with the mandibular canal or mental foramen. A DPR study has shown that 2% of mandibular second molars are radiographically related to the mandibular canal.¹⁵ This underlines the importance of taking pre-operative dental radiographs as an initial screening tool to identify potential risk of sensory nerve deficit.

Important factors to consider on radiographs when predicting injury to the inferior alveolar nerve include deflected roots, darkening of the root as it crosses the canal, narrowing of the roots, decorticated margins of the canal and diversion of the canal.¹⁶ The complex root anatomy and decortication of the mandibular canal associated with LR7 on the pre-operative DPR highlight that an increased risk for sensory nerve deficit could have been anticipated in this case (Figure 2).

Although dental radiographs can identify proximity of roots to the canal, they are unable to reveal the precise anatomical relationship.

CBCT is considered the best diagnostic tool to assess the surrounding dental anatomy. It has higher sensitivity (93% vs 70%) and specificity (77% vs 63%) than radiography, lower radiation compared to traditional CT, and is less susceptible to metal interferences to image quality than MRI.^17,18

Risk of nerve injury should be evaluated after taking pre-operative radiographs, and, where a close relationship is observed between teeth and the mandibular canal, supplementary 3D imaging such as CBCT is advised. With full anatomical disclosure, this can allow for stratification of risk of sensory nerve damage, which may assist the clinician to avoid treatment, or plan a safe direction of orthodontic movement to avoid nerve injury. Despite sensory nerve damage resolving in all the aforementioned cases, warning of potential sensory nerve damage and symptoms should be included, where appropriate, to obtain fully informed consent.

In the case of third molar extractions if the inferior alveolar nerve is injured, regeneration and recovery of sensation is expected with favourable outcomes shown in the absence of displacement of the nerve.¹⁶

Excellent prognosis has been suggested where recovery of sensation is observed after 4–8 weeks when nerve injury is likely to be due to minor compression (neurapraxia).¹⁹ Recovery of sensation observed at 3 months is likely to be a result of crush, stretch or severe compression (axonotmesis).¹⁶ In absence of improvement, distant nerve degeneration usually occurs after 3–9 months and is suggestive of complete sectioning of the nerve (neurotmesis).¹⁶

Prolonged or irreparable recovery is more commonly associated with surgical interventions, such as third molar extractions and orthognathic surgery, where risk of sectioning the inferior alveolar and lingual nerve is higher.¹⁶ Altered sensation is representative of nerve dysfunction in the absence of improvement after 2 years and permanent nerve damage should be considered. Some consider this at 3 months, although full recovery has been reported up to 2 years.^16,19 Patient history, mechanosensory testing and dermatome mapping have also been suggested to monitor progress of recovery.^10,16 Where there is little or no recovery after 3 months, referral to the local maxillofacial unit for further investigation is advocated.^10,16

High-dose oral non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (800 mg TDS for up to 3 weeks) and steroids (prednisolone on a 5-day ‘step-down’ dose, 50–40–30–20–10 mg) have been recommended to minimize any inflammation, which may be a cause for the altered sensation. These should be administered with caution by taking into account associated risks, such as gastrointestinal ulceration and individual medical history.²² Despite routine use in Bell's palsy and viral infections of the facial nerve, there is little evidence to support use of steroids for the prevention of peripheral sensory nerve injuries.²³

Vitamin B complex (1, 3, 6 and 12) is widely recommended for sport injuries, with suggestions that the vitamins assist with recovery of the neural transport system and treating neuralgia and hypo-aesthesia.²² There is insufficient evidence to demonstrate their benefit for nerve repair in humans, with the majority of research conducted in animal studies.²⁴

Table 1 shows studies where serratiopeptidase and vitamin B,¹³ NSAID and vitamin B,²¹ prednisolone,^25,26 and NSAID and vitamin B²⁷ were used. There was no evidence of increased recovery time between these management options compared with cases without medical intervention.

Following cessation of orthodontic traction of the offending tooth, full recovery of sensation was seen at review within 4 weeks for all cases with the exception of Chana et al²⁰ and Pithon²¹ where full recovery was observed at 4 and 13 months, respectively. For the latter, this was presumably due to the restarting of treatment while symptoms were active.^20,21

Compression of the nerve, or neurapraxia, would be the assumed mode of sensory nerve symptoms with orthodontic treatment and, therefore, duration of symptoms should be dependent on orthodontic forces being active. In consideration of this, it may be more suitable to refer to oral and maxillofacial surgery if little or no improvement is observed after 8 weeks in these cases. Vigilance is emphasized, and referrals should be expedited where there is uncertainty or deteriorating symptoms because sensory nerve symptoms can be associated with aberrant pathology, such as cavernous haemangioma or malignant tumours encroaching on the inferior alveolar nerve.^28,29

Orthodontic movement of the offending tooth was omitted from final treatment in eight cases and the occlusion was adjusted in two of these (Table 1). Treatment was completed with inclusion of the offending tooth in eight cases, with three of these changed to extrusion as primary movement.

As full recovery was observed in all cases, completion of treatment was considered with primary movement of the offending tooth in a direction away from the mandibular canal to facilitate space for safe movement into its intended position. The authors preferred in this case to omit the offending tooth from treatment to avoid risk of further symptoms and prolonged treatment. In retrospect, it may have also been advisable to not align LR7, since UL7 and UR7 remained unerupted at debond, which would suggest primary failure of eruption.

Complications associated with leaving partially erupted teeth should also be acknowledged. These include increased caries risk due to increased plaque accumulation and change in position of unopposed teeth in all three dimensions.^30,31 Positional change is clinically insignificant in the periodontally stable patient.³¹

The authors have devised a recommended management plan for patients who develop sensory nerve symptoms during orthodontic treatment (Figure 7).

Conclusion

Sensory nerve symptoms are a rare complication that may arise during orthodontic treatment, and should be recognized as a potential risk by the prescribing clinician. This is usually associated with contact or close proximity of mandibular premolar or molar roots with the mandibular canal.

Risk stratification may be predicted with appropriate imaging and careful treatment planning. In these cases, patients should be warned and consented for the risks of sensory nerve symptoms. Patients should be advised to report any symptoms, and orthodontic movement of the offending teeth should be deactivated immediately for revaluation.

Early recognition is imperative for optimal recovery, to avoid prolonged distress for patients and to rule out aberrant pathology.